This invention relates to a process for producing ferromagnetic metal particles, and more specifically to a process for heat-treating starting ferromagnetic particles or precursors thereof to convert the same into ferromagnetic powder of improved magnetic properties.
The present invention contemplates the treatment of either ferromagnetic metal particles or precursors thereof. The metal particles to be treated are of a ferromagnetic metal or alloy powder obtained by a wet reduction technique. The precursors may be an oxide or oxides or a hydroxide or hydroxides of a ferromagnetic metal or metals. Useful precursors include the oxides and hydroxides of ferromagnetic metals partially reduced by a dry reduction process.
Heretofore, acicular gamma-ferric oxide has been most commonly used as ferromagnetic particles for the magnetic recording medium. In recent years, growing demand for high density recording has led to the introduction of the magnetic recording medium using gamma ferric oxide doped with cobalt oxide or employing acicular chromium dioxide or the like. However, there are still increasing requirements for recording in even higher density.
To satisfy these requirements, development of fine metal particles as ferromagnetic particles for practical applications has been eagerly called for. Generally, for the production of fine metal particles wet and dry reduction processes are in wide use.
The manufacture of magnetic metal particles by the wet process usually consists in dissolving a metal salt in water and then adding thereto a solution of a water-soluble reducing agent in a magnetic field. The ferromagnetic particles thus obtained are fine enough to give a magnetic recording medium with a good squareness ratio. However, the product tends to have a disadvantage of low remanent magnetization, because the particles, synthesized in water, are sometimes partially converted into an oxide or hydroxide and, where sodium borohydride is employed as the water-soluble reducing agent, the magnetic powder can be contaminated with the boron compound.
A way of eliminating this disadvantage is to heat-treat the magnetic powder in an atmosphere of a reducing gas and thereby increase the remanence.
However, the magnetic powder so obtained in the form of super fines often undergoes aggregation of the particles during the heat treament. If the heat-treating temperature is lowered to avoid this danger, adequate residual magnetization will not result.
In the process of heat treatment, the reaction temperature must understandably be high enough to increase the residual magnetization and enhance the reaction efficiency. The high temperature, however, causes aggregation of the material particles, unfavorably affecting the magnetic properties of the powder and therefore the electromagnetic conversion characteristic of the resulting magnetic recording medium.
On the other hand, for the dry-process manufacture of fine metal particles, reduction with heat of an oxide of a metal, such as cobalt or nickel, or of a metal salt of an organic acid in a reducing gas atmosphere is commonly in practice. The largest obstacle in the way of commercial acceptance of the dry reduction process is the deterioration of the magnetic properties of the product with a rise in the efficiency of heating reduction and also due to the aggregation of material particles during the reduction reaction. The higher the temperature for the heating-reduction treatment, the greater the reaction rate and the productivity, with improved remanent magnetization and other magnetic properties. At the same time, the elevated temperature is apt to entail the danger of particle aggregation, which in turn will deteriorate the magnetic properties of the product.